How do I get a random number with a negative number in range?

Question

Consider the following code:

int rand = new Random().nextInt((30 - 20) + 1) + 20

It will return a random number between 30 and 20. However, I need its range to include negative numbers. How would I include negative numbers in the generation?

I have tried using math that would be negative, but that resulted in an error. Simply subtracting or adding the negative numbers would not yield the desired value.

Sorry, I am only half awake. The correct code is int rand = new Random().nextInt((30 - 20) + 1) + 20;.

"It will return a random number between 30 and 20" - nope... — Nir Alfasi, Jan 16 '15 at 03:28
@alfasin I was going to say the same thing. Also, OP what range is the range of numbers you're looking to have? — David.Jones, Jan 16 '15 at 03:30
@PM77-1 Except, as in this case, when it's `11` (exclusive) or `10` (inclusive). — Elliott Frisch, Jan 16 '15 at 03:39
*Simply subtracting or adding the negative numbers would not yield the desired value.* Technically true, it yields a desired range. What is the desired range? — Elliott Frisch, Jan 16 '15 at 03:44
30 - -10. I'm trying to make a survival simulator. That will be my temperature. — CaffeineToCode, Jan 16 '15 at 03:44
Possible duplicate of [How to generate random integers within a specific range in Java?](https://stackoverflow.com/q/363681/608639) — jww, Jan 02 '19 at 17:59

score 32 · Accepted Answer · edited Sep 04 '20 at 18:03

32

To get a random number between a set range with min and max:

int number = random.nextInt(max - min) + min;

It also works with negative numbers.

So:

random.nextInt(30 + 10) - 10;
// max = 30; min = -10;

Will yield a random int between -10 and 30 (exclusive).

It also works with doubles.

edited Sep 04 '20 at 18:03

Peter Mortensen

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answered Jan 16 '15 at 03:55

EDToaster

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Doesn't work when the range `max-min` is greater than `Integer.MAX_VALUE` – awwsmm Aug 09 '19 at 08:26
@awwsmm you can cast it to a long or something – EDToaster Aug 09 '19 at 08:27
Returns numbers out of range when negative. – Epic Speedy Nov 14 '21 at 17:39

score 15 · Answer 2 · edited Sep 04 '20 at 18:07

15

You can use Random.nextBoolean() to determine if it's a random positive or negative integer.

int MAX = 30;
Random random = new Random(); // Optionally, you can specify a seed, e.g. timestamp.
int rand = random.nextInt(MAX) * (random .nextBoolean() ? -1 : 1);

edited Sep 04 '20 at 18:07

Peter Mortensen

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answered Jan 16 '15 at 03:38

KennyC

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score 1 · Answer 3 · edited Sep 04 '20 at 18:06

Okay. First, try to only create the Random instance once, but for an example,

int rand = -15 + new Random().nextInt(31);

is the range -15 to 15.

The Random.nextInt(int) JavaDoc says (in part) Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive). Note that your provided example of (30 - 20) + 1 is the range 0 to 10 (inclusive).

As a further example, to get the range 20 to 30 you would use:

int rand = 20 + new Random().nextInt(11);

Remember, the bounds of the result of is 0 to n.

30 - -10. I'm trying to make a survival simulator. That will be my temperature.

Ok. Let's write that range as -10 to 30. nextInt(n) will return a value between 0 and n, so if you want the range below 0 you must subtract 10 from the result and add 10 to the n. That's

Random random = new Random();
int rand = random.nextInt(41) - 10;

Now let's examine how we can determine those numbers. Remember, nextInt() will return between 0 and n (exclusive) and we want -10 to 30 (inclusive); so 41 is n and we subtract 10. If the result is 0 (the min) we get -10, if the result is 40 (the max) we get 30.

score 0 · Answer 4 · answered Jan 16 '15 at 03:30

0

int randomNumber=(random.nextInt(DOUBLE_MAX)-MAX);

Alternatively, create a random positive or negative 1 and multiply it.

answered Jan 16 '15 at 03:30

jnd

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How do I get a random number with a negative number in range?

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